Assessing
Biomass Feasibility

Currently Prevalent Biomass Combustion
Technology

Introduction

By far the most common means of converting biomass
to usable heat energy is through straightforward combustion, and this
accounts for around 90% of all energy attained from biomass. There are
a number of different technologies available that can be used for biomass
combustion and the main ones can be categorised under two headings: Fixed
bed combustion systems and fluidised bed combustion systems.

Fixed Bed Combustion

There are two prominent types of fixed bed combustion:
underfeed stokers and grate firings. With these methods of combustion
air is primarily supplied through the grate from below, and initial combustion
of solid fuel takes place on the grate and some gasification occurs. This
allows for secondary combustion in another chamber above the first where
secondary air is added.

Underfeed Stokers
Generally only suitable for small-scale systems, underfeed stokers are
a relatively cheap and safe option for biomass combustion. They have the
advantage of being easier to control than other technologies, since load
changes can be achieved quickly and with relative simplicity due to the
fuel feed method. Fuel is fed into the furnace from below by a screw conveyor
and then forced upwards onto the grate where combustion process begins.
Underfeed stokers are limited in terms of fuel type to low ash content
fuels such as wood chips. Due to ash removal problems it is not feasible
to burn ash rich biomass as this can affect the air flow into the chamber
and cause combustion conditions to become unstable.

Grate Firings
There are several different types of grate firing, with both fixed and
moving grates commonplace. They have the distinct advantage over underfeed
stokers in that they can accommodate fuels with high moisture and ash
content as well as with varying fuel sizes. It is very important that
fuel is spread evenly over the grate surface in order to ensure that air
is distributed uniformly throughout the fuel and thus combustion is kept
homogenous and stable. There are a number of different types of grate
firing including fixed grates, moving grates, rotating grates and travelling
grates.

Fluidised Bed Combustion
Systems

Fluidised bed furnaces operate in quite a different
manner from fixed bed furnaces and have a number of advantages associated
with them. There are two main types of fluidised bed furnace, Bubbling
Fluidised Bed (BFB) and Circulating Fluidised Bed (CFB).

Bubbling Fluidised Bed (BFB) Furnaces
The fundamental principle of a BFB furnace is that the fuel is dropped
down a chute from above into the combustion chamber where a bed, usually
of silica sand, sits on top of a nozzle distributor plate, through which
air is fed into the chamber with a velocity of between 1 and 2.5m/s. The
bed normally has a temperature of between 800 and 900°C and the sand
accounts for about 98% of the mixture, with the fuel then making up a
small fraction of the fuel and bed material.

BFB’s have two main advantages in terms of fuel
size and type over more traditional fixed bed systems. Firstly they can
cope with fuel of varying particle size and moisture content with little
problem, and secondly they can burn mixtures of different fuel types such
as wood and straw. BFB’s are only a practical option with larger
plants with a nominal boiler capacity greater than 10 MWth.

Circulating Fluidised Bed (CFB) Furnaces
If the air velocity is increased to 5-10m/s then a CFB system can be achieved,
where the sand is carried upwards by the flue gases and a more thorough
mixing of the bed material and fuel takes place. The sand is then separated
from the gas in a hot cyclone or U beam separator at the top of the furnace
and fed back into the combustion chamber where the whole process begins
again.

CFB’s deliver very stable combustion conditions
but it comes at a cost. Due to their larger size compared to other combustion
methods the cost is relatively high and there are problems involved with
fuel size, which must be very small, and the difficulties involved in
running them at partial load. All of this means that they are really only
feasible for plants with a boiler capacity of over about 30MWth.